A ceramic multi-layer device, such as a small filter or a module component, is often manufactured by stacking ceramic green sheets one over another. During this manufacturing, the ceramic green sheets are provided at their surfaces with patterns of wiring by screen printing the patterns, the sheets are assembled in layers, the sheets are baked, and the sheets are divided into pieces of ceramic multi-layer devices. The ceramic green sheets are ceramic layers to construct a ceramic multi-layer device.
Patterns of wiring provided on both sides of a ceramic layer are connected to each other by a via-electrode extending across the ceramic layer. The via-electrode is fabricated by filling a via-hole in a ceramic green sheet with paste of electrically conductive material and then by baking the paste. The ceramic green sheets may be doped with additive, such as glass, for baking the sheets at a low temperature.
As required to have a small overall size and an improved performance, the ceramic multi-layer devices are demanded to have larger number of these via-holes, made in the ceramic green sheets, having small diameters, in a shorter time.
The via-holes are made by punching the ceramic green sheets with a die. The smaller the diameters of the via holes, the shorter an operating life of the die, and pins of the die, are. This punching does not provide via-holes having diameters smaller than 100 μmφ easily, and provides via-holes at a small processing speed.
As disclosed in Japanese Patent Laid-Open Publication No.01-9691, via-holes may favorably be made by utilizing a laser beam of CO2, YAG, or excimer. This method can provide via-holes having diameters smaller than 100 μmφ easily while not requiring particular maintenance operations, such as a change of a die.
The laser beam can make the via-holes fast by repetitively irradiating a ceramic green sheet with the laser beam with a galvano-scan mirror having a reflecting angle controlled by electronic control without movement of the ceramic green sheet. This allows the via-holes to be provided at a speed substantially 100 times larger than that of the punching.
FIG. 8 is a cross sectional view of a baked ceramic assembly in a conventional method. FIG. 9 illustrates a conventional via-hole.
In this technique using a laser beam, a ceramic green sheet containing particularly glass material may include a melting portion of the glass material while being irradiated with the laser beam to provide via holes. The melting portion of the glass material appears as a melting substance 17 remaining at an edge of via-hole 16, as shown in FIG. 9.
The melting substance 17 prevents the via-hole 16 from being filled with paste of electrically conductive material at a succeeding process for fabricating via-electrode 12. Portions around the via-hole 16 in the ceramic green sheet may deteriorate by heat of the laser beam, hence significantly deteriorating a joining tightness and wet property between the electrically conductive material paste and the ceramic green sheet.
A baked ceramic assembly 10 is provided by forming the via-holes 16 in the ceramic green sheets by utilizing the laser beam, filling the via-holes 16 with the electrically conductive material paste, assembling the sheets in layers, and baking the sheets. As shown in FIG. 8, the baked ceramic assembly 10 may have a structural defect 13, such as a cavity, around via-electrode 12. This defect reduces a connecting property as well as resistance to moisture, thus reducing long-term operational reliability of a manufactured ceramic multi-layer device.